1. Field of Invention
This invention relates to a guide arrangement for marine risers, in particular for offshore oil and gas operations.
2. Description of Related Art
Risers are often used for connecting offshore floating platforms or vessels with subsea installations. Such risers may be of various kinds, for example electric cables, fluid pipes, umbilicals or other forms of combined risers being of a flexible character. Typically, these risers are provided at intermediate portions of their length with buoyancy elements so as to obtain a favourable total curve or trajectory of the risers through the water.
In some cases where dynamic conditions have to be taken into consideration and there is limited space available around the floating platform or vessel concerned, there is a requirement for stabilizing or anchoring the risers so as to reduce or avoid sideways motions thereof. Such motions may be caused by sea currents or waves as well as other influences acting on the risers and/or platform/vessel.
In a known solution (Subsea Arch System by CRP Group Limited, Lancashire, England) to the above problem there is provided                at least one guide structure for a length of riser,        a frame assembly for supporting said guide structure,        anchor means at the seabed,        tether means connecting said frame assembly to said anchor means, and        a buoyancy element for keeping said guide structure at a desired level in the sea during operation.        
More specifically, the known solution involves the use of a fixed guide structure in the form of an arch having an upward or “convex” curvature when installed for stabilizing one or more risers. However, the combination of a light and flexible riser, such as an umbilical or the like, with large dynamic movements as explained above, will require very large bending stiffeners at the entrance and exit of the arch. This involves highly increased costs.